Well, in 1941 18 exposures of 135 Kodachrome were $2.68. It would take at least 2 rolls to be as much film as one 8x10, plus being hand-processed the 8x10 must have cost more. Lets assume $7 a sheet. That's over $80 corrected for inflation today.
(That 18 exposure roll of Kodachrome is almost $39 corrected for inflation. Kodachrome was for the rich then.)
Of course, in 1945 you had to "know someone" to get some Kodachrome...

John,
if you look further down the page, you can see that Kodachrome sheet films are listed, so you don't have to guess at the price. 8x10 was $18.00 for 6 sheets, which divides out to $3 a sheet. Based on official inflation rates, $3 in 1941 is $44.94 today.
For comparison, Badger Graphic is selling E100G in 8x10 for $97.95 per 10-sheet pack, or $9.80 a sheet. Add to that $5.00 per sheet processing (from Samy's), and you end up with $14.80/sheet. So, adjusted for inflation, 8x10 Kodachrome was 3x more expensive than 8x10 today.

That price list shows 6 sheets of 8x10 Kodachrome for $18 or $3 per sheet, or about $45 per sheet adjusted for inflation. It sounds as if Mr. Kelly used one box (about $270) in order to give Ms Monroe $50. Nice guy.